Abstract
The current design practice of composite material aeronautical structures imposes the use of knock-down structural material allowables to take into account the high sensitivity to environmental exposure (i.e., moisture, temperature, damages). The “moisture derating factor” comes from specific mechanical test campaign and drastically reduces the advantage of using such materials; but the continuous monitoring of the moisture content of the structure could enable the use of higher design allowables. In the framework of FUSIMCO (Work developed within the frame of the Project FUSIMCO-FUSoliera Ibrida Metallo COmposito-co-financed by MIUR-Italian Ministry of Research with DAC-Campania Aerospace District as beneficiary and Leonardo Company-Aerostructure Division as “prime” partner) project, the aim of this study is to verify the effectiveness of the impedance measurement method as a health-monitoring tool to evaluate the moisture quantity absorbed by an aeronautical composite structure. The method is based on the idea that a composite laminate can be associated with an equivalent electric circuit (EEC). Some electrical characteristics of this EEC can be associated to the moisture content of the laminate. A simple EEC model, mainly capacitive, was used. A frequency sweep was the electric stimulus signal of some electrodes, glued onto the specimens to investigate the EEC parameters variation with respect to the induced moisture content variation (gravimetrically determined). The study confirmed the possibility of effectively using the impedance measurement method as a health-monitoring tool for moisture content evaluation of a composite laminate.
Highlights
The current trend in aeronautics shows an extended use of composite materials for the design of primary airframe structures
The use of higher allowables than the standard practice in the design phase is allowed by the continuous identification of the damage occurring into the composite structure and continuous measuring of the moisture absorption level by means of a dedicated sensor-network
The moisture absorption level of a composite material structure is very important because it is strictly related to its mechanical strength
Summary
The current trend in aeronautics shows an extended use of composite materials for the design of primary airframe structures. The effect of considering a structure damaged at BVID level with a moisture content reduces drastically the advantage of using such materials that have very high specific strengths with respect to the aeronautical current practice manufacturing materials This was performed so that the knowledge of the moisture absorption level of the structure during aircraft operations could enable the use of higher design allowables. A carbon fiber composite material laminate (CML), widely used in airframe structures, is an electrically conductive element (carbon fiber) within a dielectric matter (epoxy resin), so, from an electric point of view a CML specimen could be seen as an electric circuit with some basic elements, namely capacitors and resistors, connected by a net. Surface was not conductive (a sufficiently thick layer of insulating epoxy resin was present) Lateral planes were conductive, probably due to the cutting method that connected together the carbon fiber of the various layers
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